Production of high-purity tantalum flake powder

ABSTRACT

The present invention relates to a high-purity tantalum flake powder, produced by a hydride-dehydride process including: (a) cold working tantalum metal into a thin sheet; (b) hydriding the thin sheet, forming a brittle tantalum foil; (c) adjusting the tantalum foil to a desired particle size; and (d) removing hydrogen from the tantalum foil by vacuum sintering, forming a tantalum flake powder. In accordance with the present invention, tantalum flake is produced by sizing ultra-thin tantalum foil via the hydride-dehydride process. Tantalum is an extremely malleable metal and can be cold worked into extremely thin sheets less than 1 micron thick. Once hydrided, this foil is brittle, and can be easily sized by suitable milling processes. The hydrogen is removed by vacuum sintering, resulting in an extremely thin Ta metal flake.

FIELD OF THE INVENTION

The present invention relates generally to methods of manufacturingtantalum flake for high CV/g/high voltage capacitors, using a process ofmechanically flattening a nodular powder. Such an alternative methodresults in the production of tantalum flake with the potential forhigher capacitance.

BACKGROUND OF THE INVENTION

Tantalum electrolytic capacitors date back to the late 1940's. Sincethat time, many improvements have been made to the manufacturingprocess, allowing for smaller, more reliable, and better performingcomponents. Today, high CV/g/high voltage tantalum capacitors aredominated by flake technology. The particular flake morphology allowsfor high voltage applications, as the contacts between particles areline, rather than point with traditional powder metallurgy. As thedielectric is grown on the tantalum, it consumes a portion of theunderlying metal. The thicker the dielectric, the more metal isconsumed, resulting in thinner contacts between particles. The entiresinter neck eventually is consumed and the particles become electricallyisolated. Flake morphology allows the dielectric to be formed to highervoltages before choking off the sinter necks between particles.

Currently, tantalum flakes are typically produced by mechanicallyflattening tantalum particles. The particles are either from tantalum EBingots or from the reduction of K₂TaF₇ with Na metal in a molten saltreactor. The malleable metal particles are flattened in a high-energyball mill before being hydrided and reduced in size by impact milling.In order to reduce contamination, the tantalum is usually ball milled inan organic solvent, and acid leached. A deoxidation step is needed toreduce the oxygen to suitable levels for capacitor use, and a heattreatment is then applied to produce the necessary physical propertiessuch as flow, and Scott Density.

The mechanical flake process produces a distribution of particlethickness. This variation in flake thickness reduces the performance ofthe capacitor at a given formation voltage. Flakes thinner than aspecific value will be completely choked off, while flakes thicker thanthat value will not, for a given forming voltage. Capacitor powdermanufacturers are continually improving the manufacturing process, in anattempt to reduce distribution of flake thicknesses. The presentinvention relates to a process to produce tantalum flake suitable foruse in high CV/g/high voltage capacitor applications that overcomestechnical difficulties in existing processes.

SUMMARY OF THE INVENTION

The present invention relates to a high-purity tantalum flake powder,produced by a hydride-dehydride process comprising:

-   -   (a) cold working tantalum metal into a thin sheet;    -   (b) hydriding the thin sheet, forming a brittle tantalum foil;    -   (c) adjusting the tantalum foil to a desired particle size; and    -   (d) removing hydrogen from the tantalum foil by vacuum        sintering, forming a tantalum flake (agglomerate) powder.

In accordance with the present invention, tantalum flake is produced bysizing ultra-thin tantalum foil via the hydride-dehydride (HDH) process.Tantalum is an extremely malleable metal and can be cold worked intoextremely thin sheets less than 1 micron thick. Once hydrided, this foilis brittle, and can be easily sized by suitable milling processes. Thehydrogen is removed by vacuum sintering, resulting in an extremely thinTa metal flake. In embodiments, the hydriding occurs at a temperature ofabout 400-800° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention as well as other information pertinent to the disclosure, inwhich:

FIGS. 1 and 2 are illustrations of tantalum flake produced by the priorart mechanical flake process;

FIGS. 3 and 4 are illustrations of dehydride tantalum flake produced bythe process of the present invention, from 0.5 μm foil;

FIGS. 5 and 6 are further illustrations of dehydrided and hydridedtantalum flake, respectively, produced by the process of the presentinvention;

FIGS. 7 and 8 are illustrations of hydride tantalum flake produced bythe process of the present invention, from 2.5 μm foil; and

FIGS. 9 and 10 are illustrations of hydride tantalum flake produced bythe process of the present invention, from 25 μm foil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An advantage of the process of the present invention over the mechanicalprocess is that the flakes produced have a much narrower thicknessdistribution. The final flake thickness is completely dependent on thefoil thickness prior to hydriding. Foil thickness can be measured andselectively chosen to produce an extremely narrow finished flakethickness distribution. With the mechanical flake process, flakes thatare too thick or too thin cannot be removed from the sample. The methodof the present invention is an improvement in the mechanical flakeprocess.

An additional advantage of the process of the present invention is theattainment of uniform thickness across the length of each individualflake. In other words, the flake is the same thickness at the center asit is at the edge. This is not the case with the mechanical flakeprocess, where flakes tend to be much thinner at the edge than at thecenter. The advantage of a uniform flake thickness is more uniform,thicker sinter necks between particles.

Since the tantalum foil is cold rolled into thin sheets, the process ofthe present invention has the potential for lower levels ofcontamination than the mechanical flake process. Note that mechanicalflake manufacturers mill tantalum in organic solvents to minimizecontamination; a consequence is an increase in the carbon content of thefinished material. Carbon is presumed to be a significant cause of fieldcrystallization during the forming process, which leads to an increasein the leakage current of the capacitor. It is anticipated that othercontaminants will be lower with the flake process of the presentinvention, including oxygen.

EXAMPLES

Several sheets of Ta foil at various thicknesses and foil dimensions(150*500 mm, 50*50 mm and 25*25 mm) were tested, as shown in Table 1,below.

TABLE 1 Sample Foil Thickness (μm) Foil Dimensions (mm) 1 25 150 × 500 22.5 50 × 50 3 0.5 25 × 25

Foil samples 1 and 2 were hydrided in a large HDH vessel at 600° C.Sample number 3 was hydrided in a small HDH vessel at 600° C. afterbeing annealed under vacuum at 900° C. for 24 hours. After hydriding,each sample was broken apart using a mortar and pestle. Images weretaken on an ISI SR-50 Scanning Electron Microscope with secondaryelectron detector. Samples 2 and 3 were then dehydrided in a furnace at700° C. and 600° C. respectively.

SEM images of the mechanical flake process are shown in FIGS. 1 and 2.These images show the flakes prior to hydriding, and final sizing. Thefinished flakes will be much smaller in size. Edges of the flakes appearsharp, and less than 1 micron thick. FIGS. 3 and 4 show dehydridetantalum flake from the flake process of the present invention, at thesame magnification. The foil used to produce these flakes was 0.5 μmthick (Sample 3). As shown in these images, the process of the presentinvention produces tantalum flakes very similar in size and shape to themechanical flake process. By introducing a suitable classificationprocess, most of the “fines” present in the material produced inaccordance with the present invention could easily be removed.

As noted above, the process of the present invention results in moreuniform thickness across the length of the individual flakes, whichtranslates into thicker edges on the flakes. The edges of the flakes aretypically where the sinter necks are formed. A thicker edge providespotential for thicker sinter necks. The SEM images of FIGS. 5 and 6 arehigh magnification flakes produced in accordance with the presentinvention, which show the well-defined edges. FIG. 5 shows dehydridetantalum foil, while FIG. 6 shows hydrided tantalum foil. Both are from0.5 μm foil (Sample 3). The difference in the thickness of the flakes isattributable to the difference in density between hydride and dehydride.Tantalum hydride has a density of about 13.2 g/cm³, while tantalum metalhas a density of 16.6 g/cm³; this represents a 26% difference indensity, which corresponds to a 26% increase in the flake thickness fromhydride to dehydride. The measured thickness difference in the SEMimages is 24%, which shows a good correlation to the theoretical.

Tantalum flake produced from thicker tantalum foil (Samples 1 and 2)were also imaged on the SEM. These images illustrate the dependence ofthe initial foil thickness on the final flake thickness. They also showthat a hydrided tantalum foil is more likely to break perpendicular tothe foil surface, rather than parallel to it. The first set of images(FIGS. 7 and 8), is hydrided tantalum flake from 2.5 μm thick tantalumfoil (Sample 2). This sample was not annealed prior to being hydrided,as the 0.5-μm sample (Sample 3) was in FIGS. 3 and 4. The high surfacestresses from the cold working, when not relieved by thermal annealing,tended to produce more irregular shaped flakes when sized as hydride, asshown in FIG. 8. The flake produced from the 25 μm foil does not have asmuch surface stress, and therefore produced more uniform flakes. Thesecond set of images (FIGS. 9 and 10), are hydrided tantalum flakeproduced from 25-μm thick tantalum foil.

Chemical analysis from the foil samples is listed in Tables 2 and 3,below. While no carbon level is listed for the thinnest tantalum foil(0.5 μm), the level for the thickest (25 μm) is less than 10 ppm on thecertificate of analysis. This level of carbon contamination is asignificant improvement over the mechanical milling process. Minimalcarbon pick-up is expected from the HDH, and sizing processes. Othercontaminant levels listed on the respective certificates of analysis arelow enough for use in electronic applications.

TABLE 2 Specifications for tantalum foil (0.5 μm thick) Element ppm Al 5Ca 2 Co 1 Cr 5 Cu 2 Fe 30 Mg 5 Mn 2 Mo 100 Na 10 Nb <500 Ni 3 Si 10 Sn 2Ti 20

TABLE 3 Specifications for tantalum foil (25 and 2.5 μm thick) Elementppm C <10 Fe 4 H <5 Mo <2 N <10 Nb 10 Ni <1 O 14 Si <10 Ti <1 W <1

Thus, in accordance with the present invention, tantalum flake suitablefor use in high CV/g/high voltage capacitor applications can bemanufactured by hydriding and sizing very thin tantalum foil, where theinitial foil thickness determines the final flake thickness. Hydrogencan later be removed by vacuum annealing.

There are several advantages to using foil over mechanically flatteningtantalum particles. First, it produces a very narrow flake thicknessdistribution. The thickness of the flakes determines the maximum formingvoltage of the material. A higher formation voltage corresponds to athicker dielectric oxide and higher breakdown voltages. With a narrowthickness distribution, fewer flakes will be formed through for a givenvoltage, increasing the material's CV/g. In order to optimize theperformance of the material, capacitor manufacturers desire the highestformation voltage possible, without electrically isolating particles byconsuming the underlying metal during forming.

Secondly, more uniform thickness across the length of the flake isproduced. This can be seen in the sharp, well-defined edges of theflakes (see e.g., FIGS. 5 and 6). Since particles typically sinter atthe edges, a thicker edge means thicker sinter necks. Furthermore,contaminant levels are lower in the flake process of the presentinvention, particularly carbon. It is hypothesized that a major cause offield crystallization during forming is a result of carbon contaminationon the particle surface. Since the flake process of the presentinvention does not use any organic solvents during processing, carboncontamination is much lower than the mechanical flake process. Thisshould result in lower leakage current in the finished capacitors,another advantage over the mechanical process.

While the present invention has been described with respect toparticular embodiment thereof, it is apparent that numerous other formsand modifications of the invention will be obvious to those skilled inthe art. The appended claims and this invention generally should beconstrued to cover all such obvious forms and modifications, which arewithin the true spirit and scope of the present invention.

1. A high-purity tantalum flake powder, produced by a hydride-dehydrideprocess comprising: (a) cold working tantalum metal into a thin sheet;(b) hydriding the thin sheet, forming a brittle tantalum foil; (c)adjusting the tantalum foil to a desired particle size; and (d) removinghydrogen from the tantalum foil by vacuum sintering, forming a tantalumflake powder.
 2. The tantalum flake powder as recited in claim 1,wherein the flake is of a uniform thickness across its length.
 3. Thetantalum flake powder as recited in claim 1, wherein the flake has athicker sinter neck.
 4. The tantalum flake powder as recited in claim 1,wherein the hydriding occurs at a temperature of about 400-800° C. 5.The tantalum flake powder as recited in claim 4, wherein the hydridingoccurs at a temperature of about 600° C.
 6. The tantalum flake powder asrecited in claim 1, wherein the brittle tantalum foil has a thickness ofabout 0.5-25 μm.
 7. The tantalum flake powder as recited in claim 1,wherein the presence of contaminants in the powder is minimized.
 8. Thetantalum flake powder as recited in claim 1, wherein electronic valvesare produced from tantalum flake powders.
 9. The tantalum flake powderas recited in claim 1, further comprising screening the tantalum flakepowder to a final particle size distribution.
 10. A method of producingtantalum flake powder which comprises: (a) cold working tantalum metalinto a thin sheet; (b) hydriding the thin sheet, forming a brittletantalum foil; (c) adjusting the tantalum foil to a desired particlesize; and (d) removing hydrogen from the tantalum foil by vacuumsintering, forming a tantalum flake powder.
 11. The tantalum flakepowder as recited in claim 10, wherein the flake is of a uniformthickness across its length.
 12. The tantalum flake powder as recited inclaim 10, wherein the flake has a thicker sinter neck.
 13. The tantalumflake powder as recited in claim 10, wherein the hydriding occurs at atemperature of about 400-800° C.
 14. The tantalum flake powder asrecited in claim 13, wherein the hydriding occurs at a temperature ofabout 600° C.
 15. The tantalum flake powder as recited in claim 10,wherein the brittle tantalum foil has a thickness of about 0.5-25 μm.16. The tantalum flake powder as recited in claim 10, wherein thepresence of contaminants in the powder is minimized.
 17. The tantalumflake powder as recited in claim 10, wherein electronic valves areproduced from tantalum flake powders.
 18. The tantalum flake powder asrecited in claim 10, further comprising screening the tantalum flakepowder to a final particle size distribution.